The present invention relates to an Oxygen Ion Conducting/Oxygen Storage (OIC/OS) material, and especially relates to an OIC/OS material having niobium (Nb) as part of the crystal structure and which exhibits higher OS capacity and more facile OS properties compared to materials of similar Ce-content. This new material further exhibits a unique property of reversible crystal structure changes upon calcination in oxidizing and reducing environments.
It is known that ceria (CeO2) plays a number of important roles in automotive three-way conversion (TWC) catalysts for the removal of post-combustion pollutants. Among these are: stabilization of precious metal (PM) dispersion, alumina support stabilization, promotion of the water gas shift (WGS) reaction, promotion of the carbon monoxide (CO)+oxygen (O2) reaction to give carbon dioxide (CO2); the nitric oxide (NO)+CO reactions to give CO2 and N2, and finally oxygen storage (OS) properties. The oxygen storage ability of CeO2 arises due to the facile nature of the Ce4+/Ce3+ redox reaction in typical exhaust gas mixtures: the reduction of CeO2 to cerous oxide (Ce2O3) provides extra oxygen under fuel rich conditions and oxidation of Ce2O3 to CeO2 builds up an oxygen reserve under fuel lean conditions. Such a facile oxygen storagexe2x80x94oxygen release ability is important for controlling the ratio of oxidants (air(A)) and reductants (fuel(F)) (A/F ratio) in the exhaust, so that CO and hydrocarbons (HCs) can be oxidized simultaneously with the reduction of nitrogen oxides (NOx). The A/F ratio is defined as the weight of air divided by the weight of fuel. For a typical gasoline fuel, an A/F ratio of 14.5-14.7 gives an exhaust composition where there are enough oxidants (O2+NOx) to completely convert the unburnt HCs and CO in the exhaust to carbon dioxide (CO2), water (H2O), and nitrogen (N2). This is referred to as stoichiometric operation and typically occurs during a cruise or idle operation of the vehicle. During accelerations exhaust compositions with excess HCs and CO are generated (rich mixtures with A/F values less than the stoichiometric value) and during deceleration, compositions with excess oxidants are generated (lean mixtures with A/F values greater than the stoichiometric value). The facile release and consumption of oxygen is important during driving conditions that generate these A/F transients away from stoichiometry so as to prevent the break through of pollutants such as HCs, CO and NOx. Catalysts that are used in these applications are referred to as three-way-conversion (TWC) catalysts as they convert the three main pollutants (HCs, CO and NOx) to innocuous products.
In older TWC catalysts, pure CeO2 was used as the oxygen storage component. However, in the older TWC catalysts, because of poor thermal stability, a large loss of oxygen storage capacity occurs above 900-1,000xc2x0 C. Modem TWC catalysts require more durable and facile OS characteristics. This has resulted in the replacement of pure CeO2 with solid solutions based on Cexe2x80x94Zr. Unlike composite metal oxides in which a solid solution is not formed between all the components in the mixture of metallic and oxygen species present, these solid solutions typically refer to a single, substantially homogeneous, metal oxide crystallite or crystallites characterized in that the oxygen atoms in the crystal structure are attached to metal ions of more than one metallic species. These type of materials are further characterized by having a single crystal structure and are referred to as single phase materials of tetragonal or cubic crystal structure. Lower valent rare earth or alkaline earth dopants can also be present in these newer materials. These type materials have the following general properties:
a) They have much higher OS capacity than pure CeO2. This arises, as in pure CeO2, only Ce4+ ions at the surface of the crystallites are redox active. However, for Cexe2x80x94Zr based solid solutions bulk Ce is also redox active in typical exhaust gas compositions and reduction of bulk Ce results in oxygen migration to the crystallite surface where it can be used to oxidize HCs or CO. Thus, these materials are referred to here as OIC/OS type materials as their function involves both oxygen storage and oxygen mobility characteristics. These differences between pure CeO2 and Cexe2x80x94Zr based solid solutions are illustrated in FIGS. 1 and 2.
b) A further advantage of Cexe2x80x94Zr based solid solutions is that they are thermally more stable than pure CeO2. This results, after aging, in slower sintering rates or particle growth rates and higher aged OS capacity.
c) It has also been found that increasing the Zr content in Cexe2x80x94Zr solid solutions results in a lowering of the cerium reduction energy in going from Ce4+ to Ce3+ and at the same time in a decrease in the activation energy for oxygen ion mobility within the lattice. This is illustrated in FIGS. 3 and 4 from a theoretical analysis of binary Cexe2x80x94Zr solid solutions by Balducci et al., J. Phys. Chem., B., Vol. 101, No 10, P. 1750, 1997. In FIG. 3 it is further observed that the presence of lower valent ions that introduce oxygen vacancies further lower the reduction energies from Ce4+ to Ce3+. (Line A is cerium reduction energy for isolated Ce3+ and V0xe2x88x92 vacancies; B is cerium reduction energy for Ce3+xe2x88x92V0xe2x88x92 clusters; and C is cerium reduction energy for Ce3+xe2x88x92V0xe2x88x92xe2x88x92Ce3+clusters.)
d) A further advantage of Zr-rich solid solutions is that after severe aging (greater than 1,000xc2x0 C.), all the Ce in the solid solution remains accessible for oxygen storage. In contrast, only a fraction of the Ce is available for OS in intermediate Zr-content compositions. This is illustrated in FIG. 5, curve 55, where the xe2x80x9cavailablexe2x80x9d OS for aged (greater than 1,000xc2x0 C.) Cexe2x80x94Zr solid solutions of varying Zr content are plotted. The OS capacity was measured using Temperature Programmed Reduction (TPR) analysis. For this measurement the aged sample is exposed to a 5% H2/95% Ar mixture and the rate of H2 uptake is measured as a function of temperature. The fraction of Ce reduced is measured based on the following reaction:
2Ce4+O2+H2xe2x86x92Ce23+O3+H2O.
xe2x80x83The maximum available OS based on Ce content is presented as Curve 51. It is seen that Curves 51 and 55 (maximum OS based on Ce content and xe2x80x9cavailablexe2x80x9d OS) coincide only in a narrow and low range of Ce contents from 0-20 mole % Ce. An increase in Ce content above 20 mole % does not result in a corresponding increase in xe2x80x9cavailablexe2x80x9d OS higher than 0.45 millimoles per gram (mmoles/g). The consequence of this limited OS availability is that in severely aged intermediate Zr-content or Ce-rich solid solutions, only part of the Ce is redox active and capable of participating in redox reactions, whereas the rest of the Ce behaves as a structure forming element. This is true for both binary Cexe2x80x94Zr mixtures and for multi-component mixtures with other rare earth and alkaline earth dopants present.
Thus, the formation of high Zr-content Cexe2x80x94Zr solid solutions has some disadvantages. One clear disadvantage is the continuous drop in OSC capacity with increased Zr content even though these materials tend to have the best thermal stability and the most facile OS properties.
What is needed in the art are OIC/OS materials having high oxygen storage capacity while maintaining or even improving upon the thermal stability and facile nature of the redox function of Zr-rich compositions.
The present invention comprises an OIC/OS material, a catalyst comprising the OIC/OS material, and a method for converting hydrocarbons, carbon monoxide and nitrogen oxides using the catalyst. This OIC/OS material comprises: up to about 95 mole percent (mole %) zirconium; about 0.5 to about 40 mole % cerium; about 0.5 to about 15 mole % R, wherein R is a rare earth metal, an alkaline earth metal, or a combination comprising at least one of these metals; and about 0.5 to 15 mole % niobium, based upon the 100 mole % metal component in the material. The invention further comprises the reaction product of about 0.5 to about 95 mole percent (mole %) zirconium; about 0.5 to about 40 mole % cerium; about 0.5 to about 15 mole % R; and about 0.5 to about 15 mole % niobium.
The catalyst comprises: an OIC/OS material having about 0.5 to about 95 mole percent (mole %) zirconium, about 0.5 to about 40 mole % cerium, about 0.5 to about 15 mole % yttrium or optionally other earth metal; and about 0.5 to about 15 mole % niobium; a noble metal component; and a porous support wherein said zirconium, cerium, R, precious metal and porous support are deposited on a substrate.
The method for converting hydrocarbons, carbon monoxide and nitrogen oxides in an exhaust stream, comprising: using a catalyst comprising an OIC/OS material having about 0.5 to about 95 mole % zirconium, about 0.5 to about 40 mole % cerium, about 0.5 to about 15 mole % R; and about 0.5 to about 15 mole % niobium, a precious metal component, and a porous support, deposited on a substrate; exposing the exhaust stream to the catalyst; and converting hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust stream to carbon dioxide, water and nitrogen.
The above described and other features of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.